Electron discharge device



Jan. 23, 1968 J. A. RUETZ ETAL 3,365,

ELECTRON DISCHARGE DEVICE Filed Sept. 2Q, 1 963 2 Sheets-Sheet l U (\I m] 3 9 s s9 INVENTORS 9 JOHN A. RUETZ A ORNEY Jan. 23, 1 Y J. A. RUETZ ETAL 3,365,607

ELECTRON DI S CHARGE DEVI CE Filed Sept. 20, 1963 2 Sheets- Sheet 2 VENTORS J A. RUETZ RODNEY R. RUBERT FIG. 2

United States Patent 3,365,607 ELECTRON DISCHARGE DEVICE John A. Ruetz, Los Altos, and Rodney R. Rubert, Santa Clara, Calif, assignors to Varian Associates, Palo Alto, Cali, a corporation of California Filed Sept. 20, 1963, Ser. No. 310,382 2 Claims. (Cl. 315-35) ABSTRACT OF THE DISCLOSURE Improved stabilization of high frequency electron discharge -devices incorporating traveling wave interaction slow wave circuits of the coupled cavity type is achieved via the utilization of terminated resonant or non-resonant structures which contain loss or R.F. absorption means therein and which are coupled via resonant or nonresonant coupling mechanisms to the coupled cavity circuit with particular emphasis on the clover-leaf type of coupled cavity circuit.

This invention relates to high frequency electron discharge devices, and in particular to an improved slow wave structure utilized with high power microwave beam tubes.

In microwave beam tubes, such as traveling wave tubes, an electron beam is directed through a slow wave structure that serves as an interaction circuit. The slow wave structure propagates a high frequency electric field that interacts with the electron beam to decrease its velocity thereby causing bunching with resultant amplification of a high frequency input information signal. One known form of slow wave structure used with high power microwave beam tubes employs a cloverleaf assembly that affords negative mutual inductive coupling, whereby the fundamental mode is a forward wave of high interaction impedance, thereby making it suitable for traveling wave tube (TWT) operation. Such a cloverleaf assembly has been described in the Proceedings of the IRE, August 1957, pages 1112-1118; in the IRE Transactions on Military Electronics, April 1961, pages 39-45; as well as in other publications. A detailed description of the structure and operation of an electron discharge device that incorporates a cloverleaf assembly is also set forth in copending US. patent application 56,415 filed Sept. 16, 1960, in behalf of John A. Ruetz et al. and now abandoned in favor of continuation application 443,612 filed Mar. 29, 1965.

In the design of a traveling wave tube, it is necessary to match the interaction circuit to a waveguide system in order to excite the circuit at the input and extract energy at the output. In addition, for a high gain TWT it is generally necessary to sever the circuit either singly or multiply in order that the gain of a section of circuit between two severs or between a sever and a circuit to waveguide transition be sufiiciently low to eliminate feedback oscillations. However, it is known that any microwave circuit usually has a great number of propagating modes with various field configurations. In such case, it is generally not feasible to match the waveguide system to all modes of the interaction circuit. In addition, for reasons of chiciency, it is not always possible in a practical TWT device to limit the gain of each section of circuit to eliminate the possibility of oscillation for all modes that may interact with the electron beam without the addition of some other stabilizing means.

Oscillations occur in microwave beam devices when the 3,365,607 Patented Jan. 23, 1968 interaction impedance, the circuit mismatch, and the beam admittance in combination are sufficiently high. When beam power is increased in a microwave beam device, the beam admittance is usually increased rapidly thereby increasing a factor leading to instabilities. Also, an increase in radio frequency power is accompanied by an increase in beam power. Therefore, the tendency for oscillation is highest for devices operating at high radio frequency power.

In effect, instabilities may be caused by the interaction between the electron beam and a poorly terminated circuit wave. Oscillations at frequencies removed from the general frequency band of interest may cause interference problems with other R.F. equipment, may lead to defocusing of the electron beam and to distortion of the information signal being processed, as well as lower efliciency and other deleterious effects. Although it may be possible to terminate or perturb each individual mode to alter the interaction that is encountered, it is generally not expedient to affect all of the different modes in the same manner, and thus additional stabilizing means are necessary.

An object of this invention is to provide an improved slow wave structure for a high frequency electron discharge device.

Another object of this invention is to provide a novel and simple means for reducing spurious oscillations in a slow wave structure of a microwave tube.

According to this invention, the slow wave structure of a high frequency electron discharge device comprises an assembly of cloverleaf sections in tandem, each section having a peripheral wall with at least one energy coupling means therein. Energy dissipating means are coupled to the energy coupling means for extracting and expending a portion of the electromagnetic energy that appears in the cloverleaf section. The energy coupling means may be resonant or non-resonant apertures, loops or probes or combinations of these. The energy dissipative means may be a terminated waveguide or coaxial line coupled to the interaction circuit by the enengy coupling means. The energy dissipative means may also be a lossy waveguide or coaxial line resonated to give coupling at selected frequencies. The terminated resonant coupling means may be frequency selective by proper adjustment of the waveguide width or by other well known filtering techniques giving a high or band pass or band rejection response. In this manner, it is possible to select a band or range of frequencies over which stability may be improved without impairing the performance at another band or hands of frequencies.

The invention will be described in greater detail with reference to the drawing in which:

FIGURE 1 is a longitudinal view, partly broken away, of a microwave beam tube incorporating a slow wave structure;

FIGURE 2 is an enlarged cross-sectional view of a portion of the slow wave structure, partly broken away, taken along lines 22 of FIGURE 1;

FIGURE 3 is a cross-sectional view of a cloverleaf section, in accordance with the invention, taken along lines 33 of FIGURE 2;

FIGURE 4 is a cross-sectional view of a portion of the cloverleaf section taken along lines 4-4 of FIGURE 3; and

FIGURE 5 is a fragmentary view of a portion of the inventive cloverleaf section, taken along lines 5'5 of 3 FIGURE 3, illustrating an alternative embodiment of the invention.

In FIGURE 1, a traveling wave tube comprises a cathode electrode 12 from which an electron beam 14 is directed towards an anode 16, having a positive potential relative to the cathode 12. The electrons of the beam respectively traverse the anode 16 and an interaction circuit or slow wave structure 18 which includes a multiplicity of coupled cavity circuits in the form of cloverleaf sections 20, such as illustrated in FIGURES 2 and 3. The electron beam 14 is focused by a magnetic field that is uniform in the region of the interaction circuit 18 and travels axially through centrally disposed apertures 22 in the cloverleaf sections 20.

An input transducer or waveguide 24 supplies an input information signal to the input circuit 18a of the slow wave structure 18 that interacts with the electron beam 14, Whereas an output signal is derived from an output waveguide 26 coupled to the output circuit 18b of the slow wave structure. The traveling wave tube 10 also includes collector means 28 which is shielded from the magnetic field so that the beam is spread radially before collection. During operation, the input ignal is introduced to the input interaction circuit 18a through an input waveguide and is coupled from cavity 20 to cavity 20 such that the signal propagates along the circuit. The interaction between the electron beam 14 and the circuit fields causes the RF. (radio frequency) energy to experience an exponential increase with distance along the interaction circuit.

The slow Wave structure 18 comprises a plurality of sections 20 of cloverleaf configurations, one of which is shown in detail in FIGURES 2 and 3. The cloverleaf section 20 includes two metallic end Walls 32 and 34, each end wall having a circular beam opening 22 axially positioned therein, which also serves as a capacitive coupling opening between sections 20. A sinuous or four element cloverleaf-shaped metallic side wall 36 is brazed between the two end walls 32 and 34 of each section 20. The walls 32 and 34 that separate the cavity sections 20 are provided with a plurality of radially disposed inductive coupling slots 38 spaced apart every 45 relative to each other such that every other section is in alignment. A similar type of slow wave section utilized in a traveling wave tube amplifier is disclosed in US. patent application Ser. No. 7,481 entitled Inductive Coupling Means and Methods for High Frequency Apparatus, filed Feb. 8, 1960, in behalf of Marvin Chodorow, and assigned to the same assignee. The cascaded, mutually coupled cloverleaf cavities provide periodic loading of the waveguide to reduce 10. To eliminate large reflections of RF. energy from the sever section 56, a transition 58 is used to couple the RF. energy through the waveguide 40 into an external termination (not shown). An identical transition and waveguide section is used opposite the sever 55 to terminate the RF. energy from the input slow wave tructure 18a. The signal is carried across the sever space by the electron beam 14 and is reintroduced to the output interaction circuit 18b.

The cloverleaf cavities 20 are provided with channels 46 through which cooling fluids may be passed for cooling the center section of the tube 10. Additional cooling ducts and channels 48 may be provided in the slow wave structure 18 to reduce the heating effects that appear when operating high frequency tubes at high power.

In accordance with this invention, apertured portions or slots 50 (see FIGURE 4) are formed in the peripheral wall 36 of the cloverleaf sections 20, and a low loss Waveguide '52 is disposed in coupling relation with each slot 50 and cloverleaf section 20. Thus electromagnetic energy will be extracted from the cloverleafs 20 and expended in selective loss to occur at frequencies higher than those de- 7 sired to be amplified. Those frequencies below the waveguide'cut-oif would be reflected and hence not be dissipated in the waveguide termination. If it is desired to have coupling occur over a small range of frequencies, then a very narrow resonant slot or resonant aperture of predetermined dimension may be formed in the peripheral wall 36 of the cloverleaf 20.

In FIGURE 5, an alternative embodiment of the invention comprises a probe or loop 60 that is inserted through the apertured portion or slot 50 formed in the peripheral Wall 36 of the Cloverleaf section 20 and is coupled to a stop band filter 62 that discriminates against the desired signal to be amplified, and passes the RF. energy of other undesired modes for dissipation in a lossy ceramic termination 64 coupled to the filter 62. With this combination, it is not necessary to incorporate a relatively long recessed Waveguide portion adjacent to each cloverleaf cavity 20, as found in FIGURE 3.

There has been described herein a novel configuration for a cloverleaf assembly in a slow wave structure of a high power microwave beam tube. The cloverleaf has grooved or slotted portions of predetermined size in its peripheral wall, and a lossy termination is coupled to the cloverleaf cavity through the slotted portion to extract and expend electromagnetic energy thereby substantially eliminating undesirable modes.

The invention is not necessarily limited to traveling wave tubes, but is applicable to klystrons and other higher power microwave beam tubes and linear accelerators. Also, the scope of the invention is not limited to the cloverleaf configuration, shown by way of example.

What is claimed is:

1. In a high frequency electron discharge device having a slow-wave interaction circuit the improvement comprising, a slow-wave circuit for supporting high frequency electromagnetic traveling wave energy including a plurality of coupled cavities disposed along a central axis, said cavities having axially aligned central beam coupling apertures in the end wall portions thereof, said cavities havmg inductive coupling slots in the end wall portions thereof and forming a Cloverleaf circuit that provides negative mutual inductive coupling between adjacent cavities such that the fundamental mode has a forward wave, said cavities having terminated waveguides coupled thereto for extracting electromagnetic wave energy from said slow-wave circuit, said waveguides having cut-off frequencies which are above the operating region of the slow-wave circuit operating passband, said waveguides being provided with loss means for dissipating electromagnetic wave energy coupled into said waveguides from said slow-wave circuit, said waveguides being arcuate in shape.

2. A slow-wave circuit for supporting high frequency electromagnetic traveling wave energy including a plurality of coupled cavities disposed along a central axis, said cavities having axially aligned central beam coupling apertures in the end wall portions thereof, said cavities having inductive coupling slots in the end wall portions thereof and forming a cloverleaf circuit that provides negative mutual inductive coupling between adjacent cavities such that the fundamental mode has a forward wave, said cavities having terminated waveguides coupled thereto for extracting electromagnetic wave energy from said slow-wave circuit, said waveguides having cut-off frequencies which are above the operating region of the slowwave circuit operating passband, said waveguides being provided with loss means for dissipating electromagnetic 5 6 Wave energy coupled into said Waveguides from said slow- 2,970,242 1/ 1961 Jepsen 333-31 X Wave circuit, said waveguides being 'arcuate in shape. 3,221,204 11/ 19 65 Hant et a1. 315-393 X 3,221,205 11/1965 'Sensiper 31539.3 X References Cited HEQM AN KARL SAA EACH P E UNITED STATES PATENTS 5 xamme" 2,934,672 4/1960 Pollack et a1 1 333-s3 X ELI LIEBERMAN Examme 2,952,795 9/1960 Craig et a1. 333-31 X M. NUSSBAUM, Assistant Ex min r. 

